Umiko is Kou's co-worker who she likes to call by her last name which always results with her getting head-shots from the latter. The two are close enough that they engage in a drinking contest and a duel.
Rin is Kou's close friend to the point that the latter acts more like a mother to her. It is also noticeable that Rin has a huge crush on Kou which she is painfully unaware of. Possibly the only person in the company that doesn't notice Rin's feelings; as Shizuku, Hifumi and Yun are shown to be aware of it, although Aoba might be aware of it as well.
Shizuku is Kou's superior. She likes the Rin/Kou tandem in the extent that she supports, teases and even set the mood for the two.
Aoba is Kou's supervisee. She admitted that Kou's the reason for her becoming a Character Designer. She's the one who successfully persuaded Kou on becoming the Art Director.
When Hifumi started, she and Kou doesn't really talk to each other until one time when Kou called her. Hifumi replaced Kou as the Character Team Leader after admittedly said that she trust the latter.
Yun, even though being Kou's subordinate, there isn't much interaction between them. Kou entasked Yun in making 3D models for monsters.
Hajime, being a motion designer, isn't really handled by Kou until Kou become the Art Director. Kou sometimes uses Hajime's collections as a reference for character designs.